How to implement server push in Flask framework?

Have a look at Server-Sent Events. Server-Sent Events is a
browser API that lets you keep open a socket to your server, subscribing to a
stream of updates. For more Information read Alex MacCaw (Author of
Juggernaut) post on why he kills juggernaut and why the simpler
Server-Sent Events are in manny cases the better tool for the job than
Websockets.

The protocol is really easy. Just add the mimetype text/event-stream to your
response. The browser will keep the connection open and listen for updates. An Event
sent from the server is a line of text starting with data: and a following newline.

data: this is a simple message
<blank line>

If you want to exchange structured data, just dump your data as json and send the json over the wire.

An advantage is that you can use SSE in Flask without the need for an extra
Server. There is a simple chat application example on github which
uses redis as a pub/sub backend.

Server-Sent Events are supported by recent Firefox, Chrome and Safari browsers.
Internet Explorer does not yet support Server-Sent Events, but is expected to support them in
Version 10. There are two recommended Polyfills to support older browsers